1. Field of the Invention
The present invention generally relates to pulse forming networks. More particularly, the present invention relates to high-current, high-power, inductive pulse forming network for use with electromagnetic launchers such as railguns and electromagnetic aircraft launchers.
2. Description of the Related Art
A Pulse forming network (PFN) is an electrical circuit or device that provides a high-power, short-duration pulse of electrical energy of desired waveform. The elements of a PFN are electrically coupled and configured to make a transient current pulse of specific waveform across a load when a load switch is closed.
PFNs have applications, well known to those of skill in the art, in high-power, short-duration power supplies used, for example, in pulsed lasers, pulsed magnetic plasma confinement machines, high-powered microwave weapons, pulsed accelerators for flash x-ray and pulsed electron beam machines, electromagnetic launch systems, and railgun weapons.
In these applications, extremely high average power and extremely high total energy output are required for each current pulse delivered by the PFN to a connected load. To deliver high total energy, several energy storage devices for use with PFNs were previously investigated including batteries, inertial flywheels wound with coils, such as, compulsators, super-conducting magnetic energy storage devices, and high-voltage capacitors.
The United States Navy has attempted to develop a railgun with a projectile range in excess of 370 km (200 Nautical Miles), far exceeding the range of conventional shipboard weapons systems utilizing chemical propellants. In a typical design for a shipboard installed railgun application, it has been shown that between 15 and 20 Megawatts of average power over 8 milliseconds—about 160 Megajoules of total breech energy must be delivered by a PFN to accelerate a useful projectile to about 2.5 Kilometers/Second (km/s). When a railgun of this type is fired at a rate of six rounds per minute, it was shown to draw a peak power of about 20 Gigawatts, to be provided six times per minute. This large peak power requirement made the ship's bus incapable of directly supplying power to the railgun.
The operation of a railgun is straightforward. A current is passed through two metal rails electrically coupled by an armature such as a conductive projectile across the rails. The rails are not only conduits of the current but are, as well, guides within the railgun barrel for the moving projectile. The current supplied by the PFN and flowing across the conductive projectile through the rails, generates a magnetic field around each of the rails. This magnetic field acts on the electric current flowing through the projectile and the resulting Lorentz force accelerates the projectile along the rails and out of the railgun barrel.
The Lorentz force on a railgun projectile is given by:F=L′I2/2,where I is the current through the projectile and L′ is the inductance gradient of the rails, which in prior art railguns designs developed by the United States Navy was of the order of 3×10−7 Henries/meter.
Very high currents (a few million amperes) were require to accelerate sufficiently massive projectiles to the desired railgun muzzle exit speeds of about 2 to 2.5 Kilometers/Second. In a prior art capacitive PFN, high current was supplied by high-voltage parallel-coupled capacitors, in the range of about 5-20 kilovolts (kV), that were connected across the railgun load through a series coupled inductor.
After charging from a high-voltage source, the high-voltage capacitors were connected to the railgun load and were next fully discharged through the series inductor. The voltage of the capacitors was prevented from reversing by a high-current diode capable of withstanding high voltage potentials (typically >10 kV). In a manner known to those of skill in the art, in the capacitive PFN described above, pulse current through the railgun load fell, without reversing, with a time constant of L/R, where L and R are the inductance and resistance of the PFN/railgun load circuit.
The use of high-voltage capacitors in a capacitive PFN presented dangerous electrical conditions. However, putting many low-voltage (2-50 Volts) capacitors in series to supply the requisite high current did not work because the available energy was reduced and the equivalent series resistance (ESR) of the series low-voltage capacitors prevented extraction of the high currents required to drive the railgun. As is well known by those of skill in the art, real capacitors, as well as all real electrical components, include impedance associated with the departure of real electrical components from theoretically ideal electrical components.
High voltage capacitors are the most mature energy storage device available for use with a PFN. The characteristics of high-voltage capacitors are well known, they are widely used in the commercial sector, and they have among the highest energy densities, demonstrated to at least a level of 2.5 Megajoules/meter3 (MJ/m3), of any energy storage mechanisms being developed for shipboard use. High-voltage capacitors contain no moving parts and maintenance is straightforward. If a module fails, the capacitor bank that has failed is removed and replaced with a new one.
However, even at significantly higher energy densities, the space required for high-voltage capacitor banks capable of supplying the very high current and high peak and average power requirements of a shipboard railgun, are of great importance. Prior art designs for high-voltage capacitor PFNs used with railguns and other electromagnetic launchers such as aircraft carrier electromagnetic aircraft launch systems and electromagnetic aircraft recovery systems, are excessively heavy and bulky for practical application and have limited lifetimes.
Further, for railguns used as primary armament on armored fighting vehicles (AEV), where space and possibly weight are sharply restricted, no power supply suitably small and light has been developed. In addition, high-voltage capacitors used in prior art capacitive PFNs were typically restricted to a few simple geometric configurations such as square capacitors. These shapes were generally unsuitable for uses requiring atypical capacitor configurations, such as, for example, to fit along the inside of an AEV gun turret.
A current advance in capacitor design is a chemical double-layer, low-voltage capacitor, sometimes called an ultracapacitor, that has the potential to significantly increase capacitor energy density. These new low-voltage double layer capacitors are also easily configurable in a variety of complex space-efficient shapes.
However, as described above, prior art capacitive PFNs required high-voltage capacitors to supply the very high current and high peak and average power requirements of a shipboard railgun. What is needed to fully realize the performance advantages of railguns and other electromagnetic launch systems over convention systems, is a simple PFN structure and method of use that reduces the weight, space, and complexity requirements of prior art PFNs. The innovative PFN would advantageously utilize a high energy density, intrinsically safer low-voltage ultracapacitors, potentially configurable in a variety of useful, space efficient shapes, to provide a high-current, high-power pulse of desired waveform.